Several animals give calls or make other sounds in an audio frequency range which humans find to be normally undetectable or difficult to detect without aid. Perhaps best-known amongst these are bat echolocation calls and certain sea-mammal calls, though other animals are also known to use ultrasonic calls.
The typical echolocation call of a bat consists of a rapid series of short narrowband frequency-modulated whistles in the ultrasonic frequency range beyond human hearing. Calls across species vary from between 1 and 100 milliseconds in duration and between 10 and 120 kilohertz in pitch. Call rates are typically 10 per second while bats are navigating or searching for prey but can increase significantly when closing in on a meal.
The prior art includes a class of bat detectors known as time expansion detectors. These detectors typically buffer approximately 5 seconds of ultrasonic sound samples at a high sample rate after some trigger event, and then drain the buffer at a much slower sample rate (typically 10 or 16 times slower) over the subsequent 50 to 80 seconds producing output audio samples. The output is suitable for recording by a conventional audio recorder or playback through speakers or headphones. Time expansion bat detectors effectively divide the frequency information by the ratio between the input and output sample rates (e.g. by 10 or 16) and preserve all of the spectral, amplitude, and temporal information of the original signal. However, this method is not suitable for real-time bat monitoring because bat activity can not be monitored while the buffer is drained over relatively long periods of time.